Technology relating to handheld flashlights incorporating a direct current power supply in the form of replaceable batteries and low voltage, incandescent bulbs achieved a technological plateau in the 1970's. Advances in the state of the art typically related to methods of packaging the batteries and bulbs, and reflector designs. In particular, the capabilities of flashlights of this type are strictly limited by inherent characteristics of the incandescent bulb itself. Initially, evacuated bulbs using tungsten filaments enabled power supplies in the range of 1.3V (and more when such batteries are connected in series) to provide varying levels of illumination. So-called halogen bulbs permitted higher filament temperatures increasing the output of such flashlights. Nevertheless, the inherent inefficiency of incandescent bulbs limited the duration of operation of such flashlights to a matter of a few hours or less depending on the number of dry cells provided in the power supply. That is, for increased run time the batteries could be connected in parallel. For increased light intensity, the batteries could be connected in series (for increased voltage) but at the expense of run time. In addition, filament bulbs are highly susceptible to mechanical shock, breaking the filament and rendering the flashlight inoperative. In addition, substantial development effort was directed to switch mechanisms for intermittently connecting the direct current power supply to the incandescent bulb so as to render either a more reliable or inexpensive switch, or both.
U.S. Pat. No. 4,242,724 to Stone is believed to be representative of one evolutionary branch of such technology relating to the packaging of a disposable floating flashlight in which the outer casing of the light itself forms a part of the switch mechanism which, when squeezed, completes electrical continuity between two AA (1.3V each) batteries and an incandescent bulb. The flashlight is compact, and floats if accidentally dropped into water. U.S. Pat. No. 5,134,558 to Williams et al. discloses a different evolutionary branch in which the light output from four AA-type batteries is boosted by an oscillator driven transformer rectifying circuit to an intermittent high voltage applied to a xenon gas flashtube so as to provide a high intensity emergency flasher. The device disclosed in Williams et al. delivers significantly more illumination from a direct current power supply than does the incandescent bulb type of flashlight disclosed by Stone. Nevertheless, the circuitry disclosed in Williams et al. for operating the xenon flashtube is expensive, bulky, and only suitable for intermittent operation of the flashtube rather than for providing a constant light output. Thus, the teachings of the prior art disclosed by Williams et al. is not suitable for remedying the inherent limitations of the incandescent bulb type flashlight technology disclosed by Stone.
As stated above, the fundamental limitations of prior art flashlights related to inherent limitations of incandescent bulb technology, and inherent limitations of electrical circuits for driving other light generating devices such as the xenon flashtube shown in Williams et al. Nevertheless, semiconductor technology contemporarily advanced so as to provide semiconductor devices, including light emitting diodes (hereinafter occasionally “LEDs”) having significantly lower current drain than incandescent bulbs in a highly robust package operable at relatively low direct current voltages. In addition, early LEDs were substantially more power efficient than incandescent bulbs having similar current consumption characteristics. Finally, the small physical size of LEDs permitted extremely efficient packaging shapes to be adopted for such lights. U.S. Pat. No. 5,386,351 to Tabor discloses such a space efficient packaging design for a single LED flashlight. The Tabor patent discloses a two-part, snap-fit housing incorporating a discoid type battery in which one leg of a two terminal LED is employed as part of a cantilever spring switch mechanism which upon depression by a flexible button completes a direct current circuit to the LED. Unfortunately, such early stage LEDs could not provide significant light output without being driven at very high currents, in which case, the power efficiency of the LED with respect to the quantity of light produced significantly decreased. Also, LEDs in use during the period in which the Tabor patent application was filed were only capable of producing light in the red part of the visible spectrum. These two limitations resulted in an LED flashlight only having utility for intermittent operation or continuous illumination over short distances. Therefore, such personal flashlights could not supplant conventional incandescent bulb flashlights which have a more linear relationship with respect to supply voltage and current.
A high intensity incandescent bulb flashlight can be produced by merely increasing the amount of current and/or voltage supply to the bulb. Conventional LEDs, being nonlinear devices do not respond in such a linear fashion. Therefore, LEDs were often employed in lighting devices for alternative purposes, such as the color coded, multiple LED light and key device shown in U.S. Pat. No. 4,831,504 to Nishizawa et al. The Nishizawa et al. patent discloses a combination LED flashlight and key in which multiple LEDs having different colors are driven by separate, manual switches and/or a microprocessor to signal an appropriate light detecting and demodulating device in association with a door lock or operating lock. Similarly, International Patent Application No. WO 01/77575 A1 titled, “Portable Illumination Device” published on Oct. 18, 2001, to Allen discloses a unique product package for a single LED personal flashlight employing a discoid type battery in which multiple depressions of a switch incorporated into the product housing cycles the single LED through multiple modes according to instructions stored in a microprocessor within the housing. Neither the invention disclosed by Nishizawa et al. nor the invention disclosed by Allen is capable of substantially increasing the light output of the LED such that the lighting devices disclosed therein are adequate replacements for high intensity incandescent bulb flashlights. The principle reason for this is that light emitting diodes, being junction semiconductor devices, have a forward bias voltage which is predetermined by the physics of the semiconductor materials from which diodes are manufactured. The forward biased voltage of silicon-based light emitting diodes is approximately 3.6 V for aqua, blue, and white LEDs and 1.8 V for red, yellow, and green LEDs. The voltage-current characteristics of devices of this type are such that substantially increasing the applied voltage outside of a range defined by the forward bias voltage does not substantially increase the light output of the device, but merely results in vastly increased current flowing therethrough. The power output of a diode being equivalent to the product of the voltage applied thereto and the current flowing therethrough, higher voltages on the power supply side merely result in much higher current which results in wasted power without significant additional illumination. Thus, the light emitting diode can basically be characterized as a device having an optimal operating characteristic defined by a substantially constant current at a nearly fixed voltage. Therefore, the only efficient method for substantially increasing light output of a prior art LED device based on the silicon architecture is to provide multiple LEDs in parallel with the direct current voltage supply. Unfortunately, this arrangement only drains the typical (1.2, 1.5 or 3 V) battery supplies quickly until the batteries can no longer supply the forward bias voltage of the diodes. Placing the LEDs in series with the power supply merely exacerbates this problem. Thus, although the direct current power supply may be capable of providing additional current (i.e., the batteries are not fully discharged yet) the potentially depleted batteries cannot forward bias and thus illuminate the LEDs.
The semiconductor industry has recently addressed the above limitations of LEDs by providing white light LEDs based on indium-gallium-arsenic-phosphide architecture having forward bias voltages in excess of 3.6V. LEDs of this type not only provide a white light which is more effective than the red light of the prior art doped silicon technology but also produced substantially more light output for a given current. Unfortunately, the battery technology based on a voltage of approximately 1.5V per dry cell is again limited in that three dry cells in series, having a nominal voltage of 4.5V, are quickly drained to an actual applied voltage of less than 3.6V at which point the white light LED becomes inoperative even though the batteries still retain a substantial charge.
Conversely, rechargeable Nickel Cadmium battery packs having a nominal voltage of 4.5V are now commercially available. These rechargeable battery packs are not suitable for efficiently driving LEDs having forward bias voltages less than 4.5V.
Therefore, a need exists for an LED driver circuit which conditions all of the available power within the conventional dry cell battery for application to high voltage LEDs for personal lighting technology purposes.